The gdh operon, which contains the gene for glucose dehydrogenase, is developmentally controlled in Bacillus subtilis. The operon is turned on only during sporulation and only in the forespore compartment. Using beta-galactosidase as a reporter gene, we have investigated the developmental control of the gdh operon. Using deletions and site-directed mutations, we have identified the area from -10 to -35 base pairs from the start of transcription as the site which controls the onset of transcription. By in vivo and in vitro comparisons we have shown that a sporulation-specific sigma factor , which is made only inside the forespore and combines there with RNA polymerase, controls the onset of transcription. Other studies have shown that the gdh operon is not required for sporulation or germination under normal conditions. However, it is likely that germination can use alternate pathways one of which involves the operon.